Not Applicable.
The present invention relates generally to antennas and, more particularly, to reflector antennas.
Conventional reflector antenna designs require a tradeoff between high efficiency and low sidelobes. In general, the aperture illumination is tapered to minimize near in sidelobes. A xe2x88x9215 dB edge taper from the peak is a typical aperture distribution to minimize sidelobes adjacent to the main beam. However, tapering the aperture distribution reduces the illumination efficiency of the antenna aperture. For example, a xe2x88x9215 dB taper can reduce the aperture efficiency by about 25 percent and result in a 1.2 dB loss in antenna gain.
FIG. 1 shows a prior art reflector antenna 10 having a main reflector 12 that reflects energy from the feed 14. Far-out sidelobes due to so-called spillover energy 16, which exits the feed 14 but does not reach the reflector 12, and so-called edge diffraction 18, must also be taken into account.
One prior art attempt shapes a subreflector to redistribute a high taper feed pattern to almost uniform distribution on the main reflector aperture. However, with such a main reflector distribution, the near-in sidelobes are typically too high to meet standard commercial sidelobe requirements, e.g., 29-25 log10(xcex8) dBi, where xcex8 is the angle from antenna boresight.
Another attempt to provide low sidelobes and high efficiency includes synthesizing dual-shaped reflectors to produce aperture power distribution defined by 1-(1-taper)(r/a)**2, where taper is the amplitude taper, r is the radial variable, and a is the main reflector radius. This arrangement does not provide low near-in sidelobes without relatively low illumination efficiency.
It would, therefore, be desirable to provide a reflector antenna system that provides relatively low near-in and far-out sidelobes and high aperture efficiency.
The present invention provides an antenna system having a main reflector and a subreflector having geometries that optimize antenna efficiency. While the invention is primarily shown and described in conjunction with a truncated Gaussian distribution over a circular aperture, it is understood that the invention is applicable to other antenna shapes and configurations.
In one aspect of the invention, a method for synthesizing a dual reflector antenna includes selecting certain parameters for the antenna such as reflector size, feed location, sub reflector midpoint location, main reflector midpoint location, and synthesis interval. The method further includes mapping energy from a known feed pattern to a selected analytical aperture distribution. From the initial locations of the feed and reflector midpoints, the shapes of the main and sub reflectors are synthesized using wavefront parameters to determine surface normals for each surface point. The resultant reflector shapes are adjusted as necessary to correct an computational errors.
The actual aperture field distribution is modified from the initial truncated Gaussian field distribution, for example, for the final synthesis of the shaped reflectors to allow the near in sidelobes to bump against a predetermined sidelobe requirement for optimizing overall antenna efficiency.